1. Field of the Invention
The present invention relates to an image heating device which heats an image on a recording material. A fixing device which fixes the image formed on the recording material using an electrophotographic type or an electrostatic recording type or a gloss imparting device which enhances glossiness of the image by re-heating the image fixed to the recording material can be cited as an example of the image heating device. Specifically the image heating device is used for a copying machine, a printer, and a facsimile.
2. Description of the Related Art
In an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, the image is formed by transferring and fixing a toner image to a sheet. A roller fixing type is usually used as the fixing device which performs the fixing by heating and melting the unfixed toner image. In the roller fixing type, a pressure roller is pressed against a fixing roller having a heater therein, and a nip is formed to perform the fixing.
In order to output the highly glossy image, it is necessary that a time during which the sheet passes through the nip is lengthened to sufficiently melt the toner. At this point, in the roller fixing type, a roller diameter is required to be increased when the nip is enlarged, which results in upsizing of the apparatus.
When a rotating speed of the roller is decreased, the toner is sufficiently melted. However, the speed of the fixing cannot be increased.
Therefore, Japanese Patent Application Laid-Open (JP-A) Nos. 11-174878 and 05-072926 disclose an upper-and-lower-belt fixing type in which a sufficient width of pressure contact portion (length in conveyance direction) while the miniaturization and speed enhancement of the apparatus are achieved compared with the roller fixing type. In the above disclosures, the fixing device includes two opposing belt members, and the large width of the pressure contact portion is obtained by sandwiching and conveying the sheet with the belt members.
However, in the fixing device disclosed in JP-A Nos. 11-174878 and 05-072926, a region where high pressure. is generated by pressing support members against each other and a region where the pressure is absent because no support member presses the belt from both sides of the belt exist in the pressure contact portion. Therefore, when the unfixed toner image is fixed, a conveyance speed difference of the sheet is generated between the high-pressure region and the pressure-absence region. Because the belt member has flexibility to some extent, there is a problem that the belt is expanded and compressed by the conveyance speed difference to shift the image on the sheet. Further, because air and moisture in a toner layer cannot be suppressed in the pressure-absence region, image disturbance is easy to occur in the sheet such as a coated sheet in particular having low permeability.